Despite progress in maternal prenatal screening and peripartum antibiotic prophylaxis, group B Streptococcus (Streptococcus agalactiae or GBS) infection remains a leading cause of neonatal sepsis and an increasingly important cause of serious infection in elderly and chronically ill adults. Although GBS has the capacity to produce life-threatening infection in susceptible hosts, it usually behaves as a harmless commensal, colonizing the gastrointestinal or genital tract of 30% or more of asymptomatic adults. Adaptation of GBS to its human and animal hosts, and the transition from commensalism to invasive infection, is likely to involve a repertoire of bacterial responses to conditions encountered in various host environments. Preliminary studies have identified a putative two-component histidine kinase-response regulator system in GBS that controls expression of at least two prominent virulence determinants. Inactivation of the response regulator, CsrR, in two independent strain backgrounds resulted in a striking increase in the GBS beta-hemolysin/cytolysin and an equally striking reduction in CAMP factor, a secreted protein that produces synergistic hemolysis with the beta-lysin of Staphylococcus aureus. The overall objective of this project is to characterize the potential role of this novel two-component regulatory system in adaptation of GBS to the human host, both as a commensal organism and as an invasive pathogen. This goal will be accomplished through three specific aims: (1) to establish the molecular basis of transcriptional regulation by CsrR/CsrS, a novel multigene regulatory system in GBS; (2) to determine, using DNA microarrays, the effects of the Csr system on global gene regulation; and (3) to characterize the role in pathogenesis of the Csr regulatory system in vivo by studying its effects on adhesion and invasion of human epithelial cells, resistance to host immune effectors, and effects on virulence in animal models of GBS infection. Results of in vitro assays of Csr-regulated gene expression under different environmental conditions together those obtained from cell culture and in vivo infection models will link specific environmental stimuli to the functions of the Csr system in GBS adaptation to the host environment. Together, these studies will broaden our understanding of how virulence gene regulation contributes to GBS pathogenesis.